Uterine leiomyomata are among the most frequently diagnosed neoplasms among women. Although benign, these tumors represent a major source of morbidity among women of reproductive age, as they are the primary indication for nearly 200, 000 hysterectomies annually in the United States. Little is known or has been studied regarding the etiology or epidemiology of uterine leiomyomata. Accumulating clinical and laboratory evidence suggests and a major factor in the development of these neoplasms, as with endometrial carcinoma, is exposure of a woman to excess or "unopposed" estrogens. An important prediction of this hypothesis is that use of combined oral contraceptives should reduce the risk of these neoplasms. A population- based case-control study is proposed to determine hormonal and hormonally- mediated factors that may predispose a woman to develop uterine leiomyomata, with particular emphasis on the role of combined oral contraceptive agents. The setting is Group Health Cooperative (GHC) of Puget Sound, a large clinic and hospital-based pre-paid health plan serving approximately 370,000 persons. Eight hundred female enrollees 25 to 59 years of age who receive a first diagnosis of uterine leiomyomata as confirmed by surgery or ultrasound between September 1, 1995 and August 31, 1998 will be included as cases in the study, for comparison, 800 women similar in age, length of enrollment, and primary care clinic will be selected at random from computerized membership files and health care utilization files and recruited as controls. Both groups will be interviewed in-person regarding use of oral contraceptives, as well as other factors known or suspected to influence the exposure to excess estrogens (e.g. use of post-menopausal estrogens and progestogens, obesity, cigarette smoking, reproductive history). For each case and control, independent classification of use of oral contraceptives and other hormonal medications will be obtained through linkage to the GHC computerized pharmacy database. In a randam sample of 400 controls, trans-vaginal ultrasound exams will be performed to measure the prevalence of undetected leiomyomata. Analyses comparing cases and controls with respect to oral contraceptive use and other factors will be conducted to address the specific aims.